Theoretical and experimental investigation on decomposition mechanism of eco-friendly insulation gas HFO1234zeE

AC breakdown and decomposition products detection characteristics of eco-friendly insulating medium in gas insulated switchgear

Gas Insulated Switchgear (GIS) was a crucial electrical equipment that provides the safety and security of power systems, and finding the eco-friendly alternatives of sulphur hexafluoride (SF6) as insulating medium in GIS has been an urgent demand in past decades. However, few studies reported the specific dielectric strength under various electrodes and partial discharge (PD) decomposition products detection of applying the eco-friendly gas in GIS. In this paper, the insulation properties among C4F7N/CO2 mixtures, dry air and CO2 were explored under the reduced-scale gas-insulated experimental device. The relationship between AC breakdown voltage and gas pressure was gained under the different electrodes and gap distances, and 0.6 MPa 7% C4F7N/93% CO2, 0.9 MPa dry air, 0.9 MPa CO2 all possess the capacity to be the insulating medium in eco-friendly gas insulated switchgear. Furthermore, the partial discharge experimental was also carried out to realize the decomposition products detection of dry air, which designs three common defect types including metal protrusion defects, air gap defects, and metal contamination defects. The decomposition products detection result shows that the contains of CO2, CO, and NO2 linearly increase with the increasing applied voltages and times, and the partial discharge defects are distinguished according to the ratios of c(CO2 + CO)/c(NO2) and c(CO2)/c(CO). The results can provide the basis for the further development of eco-friendly gas insulated switchgear.

Diffusional Behavior of New Insulating Gas Mixtures as Alternatives to the SF6-Use in Medium Voltage Switchgear

Regarding the use of SF6 in medium voltage switchgear (MVS), a review of alternatives was encouraged by the European Parliament in Regulation No 517/2014. This is aimed at a new regulatory change, that is expected soon, which will include its prohibition, similar to what has happened with other fluorinated greenhouse gases in other fields, like refrigeration. Therefore, there is an urgent need to study the physical and chemical properties of alternative gas mixtures to determine if they are suitable to replace SF6. In this context, this work addresses the difusional analysis of new gases. Binary and ternary mixtures made of 1,3,3,3-tetrafluoropropene (C3F4H2) and heptafluoroisopropyl trifluoromethyl ketone (C5F10O), using dry air as a carrier gas, were studied. The mixtures were analyzed using original equipment, composed of UV-Vis spectroscopy technology in a sealed gas chamber, which is similar to MVS. Consequently, an experimental equipment that monitors the concentration of a gas mixture online and a model that predicts the mixing process were designed and tested. The concentration profiles were obtained concerning both the time and position in the gas chamber, and the diffusional and convectional parameters were numerically calculated and optimized in an algorithm created in Scilab.

Inhibition Effect of Solid Products and DC Breakdown Characteristics of the HFO1234Ze(E)-N2-O2 Ternary Gas Mixture

HFO1234ze(E) is an environmentally friendly SF₆ substitute gas with prominent application potential. To suppress the generation of the HFO1234ze(E) solid decomposition products, which may cause great hazards to the gas-solid insulation strength, a gas mixing scheme screening method based on the reactive force field (ReaxFF) molecular dynamics (MD) simulation was innovatively proposed. The simulation results show that the inhibitory effect of O₂ on the formation of HFO1234ze(E) solid products is better than those of CO₂ and CF₄. Further study shows that when O₂ accounts for 3.33% of the gas mixture, the solid precipitate content is reduced by 48%. The experimental study shows that an O₂ content of 3.33% can inhibit the generation of solid products by more than 50%. Besides, compared with HFO1234ze(E)-N₂, the DC breakdown voltage of HFO1234ze(E)-N₂-O₂ is slightly increased, and the breakdown voltage dispersion degree and continuous breakdown voltage drop rate are decreased. This work gives a feasible solution for the suppression of HFO1234ze(E) solid decomposition products and provides an efficient method for solving similar problems of environmentally friendly insulating gas in C/F/O/N systems.

Study on Pyrolysis Characteristics of SF6 in a Trace-Oxygen (O2) Environment: ReaxFFSFO Force Field Optimization and Reactive Molecular Dynamics Simulation

The ReaxFF_SFO force field for a SF₆-O₂ system is developed based on the density functional theory (DFT) calculation data. Then, a series of molecular dynamics (MD) simulations were performed. The results show that the main oxygen-containing compounds that appeared in the MD simulation include SOF₄, SOF₂, and SO₂F₂. The relative quantitative relationship between SOF₂ and SOF₄ can be used to determine the fault temperature. Besides, under overheating conditions, O₂ rarely undergoes a self-cracking process to generate free O atoms. Instead, the basic route for O₂ to participate in the SF₆ pyrolysis process is X + Y + O₂ = XO + YO. Furthermore, the reactivity order of various groups to O₂ is (SF₂)* > (SF₃)* > (SF₄)* > F*, so O₂ is more likely to participate in the reaction by attacking (SF₃)* or (SF₂)* groups. This study laid the foundation for the application of ReaxFF MD simulations to study the microscopic dynamic mechanism of SF₆ pyrolysis in more complex systems.